The invention relates to the heat treatment of structured polymers.
Structured particles, such as hollow particle latexes, are known to be useful as opacifying agents in coating applications such as architectural coating and paper coating. The use of structured hollow latexes in coatings reduces the need for expensive pigments, such as TiO2, without adding excessive and undesirable weight to the coatings. The hollow latex particle provides opacity in paints because the hollow center scatters light more efficiently than a corresponding solid particle. The light scattering properties of the hollow latex particle are related to factors such as: the particle wet void fraction, void size, the difference in refractive index between the particle polymer and the internal void, and the ability of the hollow particle to maintain its structure in a paint film. Gloss performance in paper coating applications is directly related to the particle wet void fraction. Hollow latexes have additional utility in areas other than coatings, such as in processes involving microencapsulation, low density bulking aids, and insulation.
U.S. Pat. Nos. 4,427,836 and 5,157,084 disclose two different processes for preparing hollow latex particles. U.S. Pat. No. 4,427,836 discloses a process for making hollow latexes by a multi-step process involving preparing a core phase composed of hydrophilic acid-containing polymers, encapsulating the core with hydrophobic shell polymers and subsequently swelling at temperatures below 120° C. with a base. U.S. Pat. No. 5,157,084 discloses a separate process for making hollow latexes by a multi-step process involving preparing a core phase comprising hydrophilic ester-containing polymers, encapsulating the core with hydrophobic shell polymers, and subsequently hydrolyzing the ester core at a temperature below 150° C. with a permanent base.
The known methods of preparing voided latexes yield particles with suboptimal performance due to the formation of structures with limited wet void fraction, and/or imperfections in the particle void and shell. In view of the shortcomings of known processes, it would be desirable to have structured polymers having superior performance, and it would be desirable to have a method for increasing the wet void fraction and/or reducing or eliminating imperfections in the particle void and shell.